80% quality steam has been employed for at least 15 years to elevate the temperature of petroleum-bearing earth formations. Other means have been employed to lower the viscosity of petroleum within formations to promote movement of this material out of the formation and into wells for producing the petroleum to the surface. However, the use of 80% quality steam is well established for this purpose.
There are several competitors manufacturing generators of steam with which to heat earth formations. Of course, there are variations in design between these generators. However, there is a common characteristic of these designs in that a fossil fuel burner is mounted within a housing whose internal walls are lined with tubes through which water and/or steam is passed to absorb radiant heat and convective heat from the flame body and hot flue gases propagated by the burner or burners.
The present problem is in the horizontal cylindrical portion of the steam generator along whose axis is propagated a flame body. It has long been the practice to mount sufficient refractory material on the internal wall of the cylindrical chamber and extend the water and/or steam tube reaches the length of the cylinder, backed by this refractory surface. Obviously, this arrangement enables the radiant heat of the flame body propagated along the axis of the cylinder to be absorbed through the tube walls and into the water and/or steam flowing through the tubes.
In the smaller sizes of steam generators (25-50 MM BTU per hr.), arrangement of the water and/or steam tube reaches is relatively simple. The tube is extended in reaches which are laid along the wall refractory close to each other, with their ends joined to form a continuous passage by welded 180.degree. bends, which bends protrude from each end of the cylindrical chamber. Thus, the water and/or steam tubes are arranged in a serpentine pattern around the periphery of the internal wall of the cylindrical chamber for uniform exposure to the radiant heat of the flame body. The water and/or steam tubes mounted in the radiant section of the generator receive their feed from upstream tubes mounted in the convection section of the generator.
As the demand for larger generators develops, some design problems emerge. The velocity of the feed water and/or steam through a single tube, or pass, in the generator can become so great that an erosion problem will develop. At the same time, with a single tube pass, the differential pressure will become too great.
In considering the total surface provided by tubes in both the radiant and convection sections of the generator, the use of the single tube pass becomes impractical. The designer finds himself juggling the foregoing factors along with acceptable skin temperatures and economical diameters of tubing available. It all becomes too much for the single tube pass construction of the smaller size generators.
For the larger steam generators, plural paths for the water and/or steam through the tubes of the radiant section are required. It is these multiple paths which raise the present problem which must be addressed by the invention. Not only must the flow into each of the separate paths be controlled, but also the absorption of radiant heat by the different tube paths must be substantially equal.
Consider the basic physical arrangement. The chamber in which the burner propagates its flame body is basically a right angle cylinder. The burner, or burners, mounted at one end of this cylindrical chamber, spew a flame body of fuel and air down the axis of the cylindrical chamber. The radiant heat of the flame body radiates to the internal walls of the chamber where the tubes for water and/or steam are mounted against the refractory lining of the internal walls. The flame body is far from a stable, well-defined, radiant body. A turbulent emission from the burner results in the flame body flickering and varying in diameter and length as it extends along the axis of the chamber. With the feed tubes divided into a plurality of groups, they will have to be related to each other as they are laid down along the internal wall of the chamber so as to absorb the radiant heat of the flame body with substantially the same quanity as between the plurality of groups.